I’ve been wondering for a while now, how the unique trap mechanism of the carnivorous plant genus Utricularia might have evolved. It seemingly represents an irreducibly complex structure and I just can’t imagine a stepwise transition from earlier forms leading to this very impressive trap.
Here’s how it works: The trap is a cup-shaped flexible leaf that it is submerged in water. It has a sealed trap door. There are glandular hairs that export water from the interior of the trap to its surroundings. This creates negative hydrostatic pressure and once the seal is disturbed by prey touching the outer trigger hairs of the trap, the door opens and the prey is sucked into the trap. Here is an illustration:
There must be antannae or trigger hairs (essentially levers) near the trap entrance
If any one of these things is absent, active suction will no longer be possible. How could a passive trap obtain all of these 5 characteristics at once or in a stepwise manner? Seems impossible to me.
It does seem impossible, at least initially, and this is an example of how rigorous study of nature deconstructs that first impression in surprising ways.
I would strongly question if any of these features are necessary. Remember, the plant only needs a small increase in nutrient uptake in order for a feature to be beneficial. Do you need digestive enzymes to get nutrients from a dead bug? No. It can just sit there and rot, releasing the nutrients the plant needs. The bladderwort ancestor would already have methods for extracting nutrients from the surrounding water, just as any other plant does. Leaves can be flexible from the start, so that’s not a problem.
The “sneaky” words here are “for the trap to function in this way”. It is entirely possible for the trap to function differently and still benefit the plant.
The leaf morphology is less in interesting to me(seems trivial to just change the shape of something), I’d be more interested in understanding the sorts of selective pressures that drove the different adaptations. I suspect a lot of this has to do with how closely related but non-carnivorous plants obtain nutrients through their leaves (from what I can gather this belongs to a family of rootless plants?). Once again, answers to these kinds of questions will only make sense in the context of phylogeny.
In all likelihood the ancestors of these plants already had numerous mechanisms for breaking down and taking up organic matter in their surroundings.
@T.j_Runyon I’ve read that paper (I even e-mailed the author), but it does not really address how exactly the bladderwort trap evolved. It merely states that the evolution of cup-shaped leafs from planar leafs could be accomplished by simple shifts in gene expression. However, it does not answer the question how water export and the sealed trap door could have evolved simultaneously (or in a stepwise fashion).
@T_aquaticus Yeah, Utricularia likely evolved from an ancestor that was already a “carnivore” with elaborate traps and enzymes. Pinguicula and Genlisea are closely related to Utricularia and they are carnivorous - however, their traps are vastly different from each other. The difficulty, so it seems, would be to explain how the Utricularia trap acquired its unique “suction” feature. Why would it begin to remove water without a sealed trap door? What good is a sealed trap door without a water export mechanism?
There are many difficult questions in science. That’s why people go to school to become scientists. The trap we have to avoid is in thinking that if we don’t know something right now that we will never know, or that it must have come about through some supernatural means. Ignorance is not evidence of anything other than our own ignorance.
This one is easy enough to envision. A sealed trap door would prevent nutrients from leaving the trap. So the plant could trap prey in a small watery compartment and seal the door, then secrete enzymes into the trap to digest the prey. As the prey dissolves in the small compartment the plant can slowly take up the trapped nutrients in solution.
@swamidass Sure, there are many carnivorous plants which do not have to transport/remove water in any way. But the morphology and functioning of the bladderwort trap are so different and so weird that it becomes hard to envision how it might have evolved. As far as I know, botanists and evolutionary biologists now assume that the common ancestor of Genlisea and Utricularia possessed adhesive leafes, like Pinguicula. How does such a thing evolve into an active trap?
@Rumraket If the sealed trap door wasn’t there from the beginning, how could the carnivorous ancestor of Utricularia have extracted any nutrients from its prey? Any nutritious fluid inside the trap would be expected to quickly flow back into the watery medium. If the sealed trap door was there from the beginning, how and why would the prey enter the trap?
I’m not sure I see a problem. Plants are masters at moving water long distances, into and out of tissues and organs. They are also capable of considerable tissue and organ fusions, as might be involved in the evolution of the trap door. Heck, anyone who has tinkered with grafting in plants knows how easy it is, in real time, for different parts of different plants to form a junction, complete with vasculature.
Most likely the same way other “rootless” plants extract nutrients from the surrounding water through their leaves/roots. After all, normal plants also exchange gases like N2, CO2, and O2, and even water through their leaves(and I know very little about plant physiology, so there’s likely other things they can do too). And they take up nutrients through their roots.
This is why I suggested earlier that to make sense of how an adaptation like this trap could evolve, we first need to inform ourselves about how plants work in general, and in particular plants more closely related to Utricularia. Not just it’s two most closely related relatives. These questions all will begin to make sense in the context of phylogeny and general knowledge of plant physiology.
If you glance at a picture of Utricularia, you will notice it’s odd morphology which basically appears to have the traps distributed in a network that basically looks like it’s “roots”:
I assume these originally evolved from more “normal” roots, which means they already possesed the ability to take up nutrients from moist soil or even from open water.
Most of it would slowly diffuse away, sure. But not all, and not instantly. Here having an intuitive understanding of physics also helps a bit. Things are just not as black and white as you make them appear. If a bug dies on a plant’s leaf, it will slowly sit there and decay away if not directly removed. Nutrients will leak into the surroundings, some small portion of which the plant will be able to take up.
The initial selection is probably simply to make the cup-shape of the trap leaf, to reduce the total surface area from which useful nutrients can diffuse away, which basically lowers the rate of loss through diffusion.
Small prey might find the cup-shaped leaf to appear to be an attractive hiding spot (maybe for nesting/laying eggs/undergoing metamorphosis in relative safety), or the plant might secrete some tasty sugary substance that lures them in. There are innumerable possibilities here.
To go on from here we have to know more details about how these plants work at the cellular and molecular level. There are mechanisms that many plants employ to regulate and change their morphology in real time in response to environmental cues (you will probably recall seeing videos of flowers closing and opening), be it light levels(time of day), temperature, humidity, strong wind etc. etc. I would guess the active trapping mechanism is homologous to some of these environmentally regulated systems, implying the evolution of the mechanism is mostly regulative.
@Rumraket I think the bladderwort traps are epiascidate leaves, not roots. I think Fleischmann et al., 2018, p. 22ff. provided a good evolutionary model for the origin of the Utricularia trap morphology:
The traps of Genlisea and Utricularia are epiascidate in ontogeny […], and so a likely scenario for their evolution is a continued inward folding and final fusion of the lateral margins of adhesive leaves of the presumed common ancestor […] These tubular leaves can be envisaged as having had an apical opening and interior surfaces covered with (carnivorous) glands. […] Such a trap system is comparatively resistant against loss of prey to rain or kleptoparasitism and the narrow tubular traps also would have worked under water.
~pp. 37-38
But again, this does not answer what came first, active removal of water or a closed trap door? You suggest that the “door” evolved first, but I have problems with this. As Jobson et al. (2004) point out:
a hermetic trap without active water removal would not function for prey capture, necessitating the pumping feature to have evolved first
Instead they argue:
In simple open traps, essential prey-derived nutrients would be expected to quickly flow back out into the environment. This result may have been a functional difficulty for the prototypical Genlisea / Utricularia trap, which we hypothesize was an open bladder system that overcame this problem with active water pumping
But this does not make sense to me either. Utricularia does not use its pumping feature to retain nutritious fluids, it actively removes water from the interior of its traps. And if the nutrients were quickly lost to the environment, then the “open bladder system” would not have been functional at all - it should’ve been an evolutionary dead end.
The traps themselves, yes. I’m not saying the trap, which is a sort of leaf, is a root. But if you simply look at the plant it looks to me like they’re attached directly on to a sort of root-system. It seems to me the plant is expressing a sort of derived leaf directly from it’s “root” network.
I’m curiously reminded of those experiments with fruit flies, where scientists were able to make flies grow antennae in place of the legs, and grow legs out of their heads instead of antennae. By altering the location of certain genes on the chromosomes, it would affect when and where particular structures would form during development.
Something analogous, genetically speaking, could be what is happening here in the plant. A developmental program is locally expressed resulting in leaves(the trap) growing directly on the root system of the plant. At least, that is what it superficially looks like to me.
Ahh okay, I see that I have misunderstood how the trap actually works. Thank you for the correction. I thought initially the trap mechanism functioned by first having the trap seal, and then some sort of pore system would expel water.
But now that I see how the system works, I think it doesn’t materially alter the scenario I gave all that much. Now the pumping mechanism could co-evolve with the initial formation of the cup-shape of the leaf. This could imply the very initial stage of the leaf’s evolution towards a cupping-shaped trap began as a sort of lure for small prey. I think this makes sense in the context of the close relatives of Utricularia being carnivorous plants with “sticky” leaves that digest prey, but on land.
Postulating an ancestor similar to those, adapting to a more aquatic environment, would entail adaptations that reduce nutrient loss when the sticky leaves periodically get wet or partially submerged. Here the cup-shape would help in the manner I described above, further aided by the simultaneous evolution of the pumping mechanism, which then was simply to reduce the rate of loss of growth-limiting nutrients.
It doesn’t use the pumping mechanism for that now. But that could be the selective pressure that drove pumping to evolve in the first place, which then inadvertently provided the basis for it’s cooption to function in the suction mechanism we see today. When the mechanism first evolved it wouldn’t have to serve the exact same function it does today.
We’ve got eyebrows made of hair, but hair did not evolve to form eyebrows, they initially evolved for temperature regulation and insulation.
But were they? If active pumping co-evolved with the cupping of the leaf, I don’t see how this wouldn’t overcome the problem. I’m not even convinced a prey organism having entered the cup and dying in there would not provide a selective advantage to the plant even if the majority of nutrients eventually diffused back out. I don’t see why we have to think it would all be pretty much instantly lost with no benefit to the plant. And we also have to consider the plant in it’s ancestral environment, which could begin mostly or fully terrestrial, then gradually adapt to perhaps a longer wet season, and then eventually become capable of permanent submersion.
Hm, I just looked at the exact functioning of the “pumping feature” and I think I now understand how this trap might have evolved. Pumping is achieved by internal glands, which also function in solute transport and digestive activities. The external glands on the other hand excrete the excess water. Secreting excess water is not uncommon in plants. This however is not the only function of the external glands:
[D]uring early stages in their ontogeny they have a role in solute absorption from the external medium.
~ Fineran (1985)
So the evolution of the pumping feature is not hard to explain: When the ancestral trap was partially or temporarily submerged in water, it acquired water pumping to prevent loss of nutrients. I’m not entirely sure whether it’s relevant or not but I note in this context the fact that those Utricularia traps which are considered to be “evolutionarily ancient” or “primitive” (like those of the section Polypompholyx) have larger xylems than their “modern” relatives.
The plant might have needed to expel the excess water at some point, for which it used the external glands of its traps (again, this might not even be the original function of these glands). And then the trap door was sealed - voilà, we have an active “suction feeder”.
Yes, all good points. And this really does highlight the importance of looking at related plants, and plant physiology in general, and put it into a phylogenetic context.
Rather than looking at some organism in isolation and trying to imagine how X structure could have evolved, which some times appears impossible at first glance, once we get more data and detailed knowledge of different and related organisms, it can help guide our thinking. We can put the organism and it’s structure in some environmental context(how might it’s ancestors have made a living, what challenges did they face?).
